Conventional jet transport aircraft typically include landing gears with anti-skid or anti-lock brake systems. One such brake system is illustrated in FIG. 1, which shows a schematic top view of an aircraft main landing gear system 100 configured in accordance with the prior art. The prior art landing gear system 100 includes a left or first wheel truck 102a and a right or second wheel truck 102b. On a typical aircraft, the first wheel truck 102a can extend downwardly from a left wing (not shown), and the second wheel truck 102b can extend downwardly from an opposite right wing (also not shown). The first wheel truck 102a can include four landing wheels 104 (shown as a first landing wheel 104a, a second landing wheel 104b, a fifth landing wheel 104e, and a sixth landing wheel 104f). Similarly, the second wheel truck 102b can also include four landing wheels 104 (shown as a third landing wheel 104c, a fourth landing wheel 104d, a seventh landing wheel 104g, and an eighth landing wheel 104h). Each wheel truck 102 can further include four wheel brakes 106 (shown as brakes 106a-h) and four wheel speed sensors 108 (shown as speed sensors 108a-h) operatively associated with the wheels 104 in one-to-one correspondence.
The landing gear system 100 can further include a wheel brake controller 110 and four processors 112 (shown as processors 112a-d). The controller 110 can be configured to receive brake control inputs from a pilot (not shown) and send corresponding control signals to the brakes 106. Each of the processors 112 can be associated with a pair of the wheels 104. For example, the first processor 112a can be operatively connected to the speed sensors 108 of the first wheel 104a and the fifth wheel 104e. Similarly, the second processor 112b can be operatively connected to the second wheel 104b and the sixth wheel 104f. While four separate processors 112 are depicted in FIG. 1 for purposes of illustration, in practice two or more of the processors 112 may be combined into a single processor that provides the same function as the two or more processors. The landing gear system 100 can additionally include an inertial reference unit 114 (“IRU 114”) configured to provide aircraft speed information to the processors 112.
In operation, the pilot initiates a brake control input from the cockpit of the aircraft. The controller 110 receives this control input, and in response sends a corresponding control signal to one or more of the brakes 106. Although a single controller 110 is shown in FIG. 1 for purposes of illustration, in some brake systems each wheel may have a dedicated controller, or conversely, the controller may be omitted and each brake may receive the control input directly from the pilot. The control input from the pilot may be an electrical signal, or it may be transmitted by actuator cable to a corresponding hydraulic valve associated with the brake 106. The brakes 106 are applied to the wheels 104 in response to the signals from the controller 110 to slow the aircraft in accordance with the pilot's control input.
Each of the processors 112 can perform routines configured to prevent the wheels 104 from locking up or skidding undesirably when the pilot applies the brakes 106. These routines can include an individual wheel anti-skid routine, a locked-wheel protection routine, and a hydroplane/touchdown protection routine. The individual wheel anti-skid routine can prevent a wheel from skidding due to overly rapid deceleration. As the brake 106a, for example, is applied to the first wheel 104a, the speed sensor 108a measures wheel speed and transmits this information to the first processor 112a. The first processor 112a monitors the deceleration of the first wheel 104a, and compares this deceleration to a maximum allowable deceleration. This maximum allowable deceleration can equate to a threshold above which the first wheel 104a would likely lock up and skid. If the deceleration of the first wheel 104a exceeds the maximum allowable deceleration, then the first processor 112a transmits a signal to the brake 106a causing the brake 106a to momentarily release. This release allows the wheel 104a to momentarily rotate freely, thus preventing wheel skid.
The locked-wheel protection routine can prevent wheel skid by preventing gross disparity between wheel speeds in a group of wheels. Referring to the first wheel 104a and the fifth wheel 104e for purposes of illustration, as the brakes 106 are being applied, the speed sensors 108 transmit wheel speed information to the first processor 112a. The first processor 112a compares the speed of the first wheel 104a to the speed of the fifth wheel 104e. If one of the wheel speeds is less than the other wheel speed by a preset amount or more, then the first processor 112a transmits a signal to the brake 106 of the slower wheel 104, causing that particular brake 106 to momentarily release. This momentary release allows the slower wheel 104 to come up to speed and prevents the slower wheel 104 from going into a deep skid during heavy braking.
The hydroplane/touchdown protection routine applies to the aft wheels 104 (i.e., the fifth wheel 104e, the sixth wheel 104f, the seventh wheel 104g, and the eighth wheel 104h) to prevent sustained hydroplane-induced wheel lockups during landing. This protection is applied only to the aft wheels 104 because these wheels touch down first during a typical landing. In this routine, the IRU 114 determines a first speed based on the speed of the aircraft and transmits this information to, for example, the first processor 112a. The first processor 112a determines a second speed based on the speed of the fifth wheel 104e as measured by the speed sensor 108e. The first processor 112a then compares the first speed from the IRU 114 to the second speed from the speed sensor 108e. If the second speed is less than the first speed by a preset amount or more, then the first processor 112a transmits a signal to the brake 106e causing the brake 106e to momentarily release. In this manner, the hydroplane/touchdown routine prevents the brake 106e from being applied to the fifth wheel 104e until the fifth wheel 104e is rotating at a speed commensurate with the aircraft speed, thus preventing wheel skid.